Just tried the site, and already the first problem is isn't as interactive as the Websites for any JavaScript framework, because naturally there is a whole contraption to make the code run into the browser.
That's odd. I have been following along here [1] and it seems just as interactive as Svelte, Angular or any of the others I've tried. There might be a few more tools that have to be installed, but that's a one time step.
Where editing the code, immediately shows the GUI related changes on the neighbouring frame.
While what I see on https://leptos.dev/ is just screenshots, and there is nothing on that documentation you linked that provides the same interactive experience, one has to explicitly start a developer sandbox to host the whole machinery, and even requires a codebox account for editing.
[1] https://book.leptos.dev/01_introduction.html